ENH: adjust wrapping routines for new vtkCellArray definition

- the vtkCellArray internal structure was still largely oriented on
  the VTK legacy format, but has now been revised.
  https://gitlab.kitware.com/vtk/vtk/merge_requests/5682

  The `VTK_CELL_ARRAY_V2` define from vtkCellArray.h indicates
  that the newer version is being used.

* In VTK-8.2.0 and older, sizes are interwoven (prefixed) in the
  connectivity.

  Connectivity: [n1, verts..., n2, verts... ]

  When using these in vtkUnstructuredGrid, also needed a secondary
  list of offsets for each of the starting locations.

* The update version now resembles a CompactListList. For example

  Connectivity: [verts..., verts... ]
  Offsets:      [0, n1, n1+n2, n1+n2+n3... ]

  The offsets are properly handled within vtkCellArray, and dropped as
  an additional input for vtkUnstructuredGrid.